Several reactor designs are being studied and proposed, with different technological solutions for the [fuels/cladding/coolant] systems.
The SFR (Sodium-cooled Fast Reactor) concept, cooled with liquid sodium (operating temperature of the coolant typically 500° C.), is the subject of an international consensus. Two opinions are envisaged for the nuclear fuel: oxide fuel (U,Pu)O2, by way of reference, and metal fuel, for example UPuZr (for example U-20Pu (20% of Pu)-10Zr (10% of Zr)), as an alternative. Imagined are, on the one hand, high-power SFR/oxide cores and small- or medium-power SFR/metal alternative cores corresponding to local and remote energy demands. For SFR/metal alternative cores, “battery” reactor designs are, for example, proposed, without refueling during the life of the reactor (lifetime fuelling), with heightened intrinsic safety requirements of the fuel.
Unlike the designs of current power-generating nuclear reactors (pressurized water reactors or boiling water reactors, with an oxide fuel) for which the fuel pin is made from metal cladding made of a zirconium-based alloy (zircaloy), the metal cladding of the fuels for sodium-cooled reactors is made of Fe—Cr or Fe—Cr—Ni stainless base for austentic or ferritic-martensitic alloys that are more or less sophisticated or improved (examples: grades EM10, T91, HT9, D9, ODS, or, more simply, 316L).
The metal fuel has particular features that pose at least three technological problems:                its swelling under neutron flux in the reactor, which creates a strong interaction that is damaging to the cladding (that is also encountered between the oxide fuels (fuel (or pellet)-cladding interaction) and zircaloy claddings, in current pressurized water reactors; a “classic” and recurrent problem). For metal fuel forms, the swelling today appears to be able to be controlled with a more resistant cladding, by an ad hoc metal fuel design and the choice of a lower density for the metal actinide alloy, so that the gaseous fission products can escape en masse into the plenum (free space left in the pin), whilst the generated and original porosity makes it possible to accommodate the deformation:        its low melting point (of the order of 1000° C.) making it a priori weaker during reactivity excursions and, more generally, temperature excursions;        the existence of eutectics (formation of a mixture of given composition during the interaction between two species or elements) between Fe, constituent element of the cladding, and U and Pu from the fuel material, with very low melting points (725° C. for the U—Fe eutectic [Journal of Alloys and Compounds 271-273 (1998) pp. 636-640], 420° C. for the Pu—Fe eutectic [Journal of Nuclear Materials 383, Vol. 1-2 (2008), pp. 112-118], of the order of 600° C. when U and Pu are alloyed with Zr or depending on the grade of the cladding), which may greatly degrade the performances of the first barrier by thinning and the margins with respect to safety. This eutectic also and finally limits the operating temperatures that it is possible to achieve, reducing the energy efficiency that could theoretically be obtained according to the laws of thermodynamics and the Carnot cycle. In the procedure for forming the eutectic, Zr has a very particular role since it makes it possible to increase the resistance to melting, and thereby makes it possible to increase the margins. Unfortunately, under neutron flux in the reactor, the Zr of the fuel migrates toward the center of the pin which is concomitantly depleted in this element in its peripheral portion.        
Instead of helium for filling the gaps and spaces between the fuel and the cladding (He-bonding), it is possible to use sodium (Na-bonding) which greatly optimizes the thermodynamics of the system. This makes it possible to reduce the temperature gradient between the center of the fuel and the cladding and to obtain large safety margins, with respect to the melting, but unfortunately hampers the procedure for the release of gaseous fission products, and complicates the management of the irradiated fuel with respect to reprocessing. Na-bonding does not eliminate the risks linked to the formation of eutectics.
One of the major problems for the use of metal fuel is therefore the formation of this eutectic and, more generally, its low melting point.
To help to resolve this problem, three major technological principles are generally proposed, studied or implemented, beyond the filling of the rod with sodium:                the modification of the composition of the fuel metal alloy;        the choice of stainless grades that represent the best compromise between mechanical strength, resistance to irradiation and increase of the eutectic temperature;        the use of metal liners, which have been the subject of many patents, some of which are commented on below.        
Indeed, there are many patents on nuclear fuel composite claddings comprising an inner liner which is usually of metallic nature, made of Zr in particular, with a diffusion barrier function, but also sometimes a specific role relating to the thermodynamics, to the internal corrosion resistance, or as a support for consumable neutron poisons, etc. Apart from a few exceptions, these patents are for the most part oriented toward use for the oxide fuel and the power-generating reactors that operate with pressurized water as coolant. Regarding the use of metal fuel, notably for the application to sodium-cooled fast reactors, patent EP 0 595 571 B1 (1997) describes the use of a composite coaxial cladding, [(outer) stainless alloy/(inner)zirconium alloy], with the inner cladding of elliptical shape, in order to optimize the thermodynamics and to minimize the occurrence of direct contact between the fuel metal alloy and the stainless cladding. The spaces created within the composite coaxial cladding may be filled with He and Na separately for optimization of the thermodynamics. It is a patent presented as an improvement of U.S. Pat. No. 4,971,753 from 1990 (EP A-0 409 405) where the concept of the composite coaxial cladding with a Zr liner is already presented. These two patents are explicitly oriented toward the use of metal fuels.
These patents refer to U.S. Pat. No. 4,894,203 (1990) where the Zr liner is modified in order to improve the internal corrosion resistance. U.S. Pat. No. 5,227,129 (1993) itself mentions the use of zirconium nitride as a liner and also a physical method for applying it. U.S. Pat. No. 5,412,701 (1995) presents the possibility of using alkali metal silicates on a zirconium base, as a support for neutron poisons.
U.S. Pat. No. 5,301,218 describes a particular technology for a liner in the shape of a rolled metal foil (several foils, like a roll of paper) and that is closed around the cylindrical fuel and welded on the outside by a particular technology (“tack welding of an inner rolled metal fuel”), everything being within the cladding. The winding turns may be deformed and become closer under the effect of a pressure or mechanical load which would be generated by the fuel.
Most of these patents directly refer, in terms of improvement, to two patents from 1996, set out in which are the technological principles of the composite coaxial cladding integrating a metallic diffusion barrier. U.S. Pat. No. 3,230,150 (1966) for the nuclear fuel UO2, has a cladding formed of an inner liner (tube) made of Cu, and from an outer part made of stainless steel, which fit together (“multi-tubular cladding”). U.S. Pat. No. 3,291,700 (1966), finally, for the metal fuel of U type or alloys thereof, notably UAlx alloys, describes a method for limiting or suppressing the interactions with the metal, notably Al, cladding. The technique presented consists in winding around the fuel, irrespective of its physical form (plate or cylinder) or its chemical nature (metallic or ceramic), a metal bandage chosen in order to completely wrap it before cladding it. The method of manufacture consists in using technological systems of simple design, made of bobbins or rollers of wrapping sheets, which distribute said material in order to wrap the fuel using given rotational movements.
For high-temperature applications (for example fourth-generation gas-cooled fast reactors [GFRs]) of these designs for advanced nuclear fuel claddings comprising linings, or more generally composite material solutions, the ceramic options are preferred, due to their more refractory character which are therefore better performing than the metal options.
Patent WO 2007/017503 (2007) describes, for example, a composite honeycombed plate design, made of monolithic and fibrous SiC, and metal liners based on refractory alloys, for nuclear fuels, for example for U, Pu carbide typically, which can operate in GFRs and that operate at very high temperature, with a very restrictive specification.
For high-temperature applications, patent application WO 2006/076039 A2 (EP 1 774 534) from 2006 is also known for an SiC—SiC composite multilayer tube that is supposed to meet the specifications of fuel elements for fourth-generation lead/lithium-cooled or gas-cooled reactors, and also for the applications of fossil-fuelled power stations. Finally, the SiC is used in an original manner as a sponge material in U.S. Pat. No. 4,710,343 (1987), for cesium notably, for a fuel pin design for a fast reactor comprising, in the upper part, silicon carbide SiC in a large extended surface area form for trapping cesium.
It emerges, in summary, that for all of the solutions proposed in the known art for the application to metal fuel for SFR reactors, a certain number of problems remain and notably:                under nominal conditions at an operating temperature of around 500° C., physico-chemical interactions between the fuel and the cladding and more particularly of the eutectic between the UPuZr metal fuel and the stainless cladding based on Fe—Cr—Ni or Fe—Cr;        under “incidental” conditions, i.e. at a temperature above the melting point of the fuel which may typically be of the order of 1000° C., the possibility of maintaining a large volume fraction in the solid state within the cladding, with good thermal properties by reducing the local neutron reactivitiy by decreasing the fissile atom concentration while promoting the maintenance of the performances of the cladding and its geometry.        
In most of the patents cited, beyond the technical effects, the questions of ease of manufacture, robustness, and cost are faced immediately for the designs of fuel pins or elements comprising liners.
For the “all ceramic” or “ceramic-metal” options in particular, the question of the assembly (manufacture, more broadly), and of the thermomechanical qualification, pose basic problems. For the “all metal” options, if the manufacturing issues are surmountable, the operational side and the amounts of metallic material to be used also pose a basic question regarding the availability and the recycling of the raw material, and therefore regarding the cost, and also regarding the methods for managing and handling fuel elements which are heavier.